!! WARNING !! Since trunk rev 33300, all lan ports including wan port acts as a dumb switch. There is no NAT. This is not yet fixed and it is related to swich functionality which was added from rev 33300. Everything is working good up to rev. 33298. So remember to compile your own working image.
The bug might be fixed in r34379 as claimed in 12125.
A working (but maybe older) firmware from this thread. This is an outdated howto.

NOTE: This method was tested on firmware 2.21b available at Edimax Poland:

Enable telnet server

Log in to www interface, lets assume it is at address http://router

In address bar go to hidden sub-page: http://router/syscmd.asp

In input box enter:

/usr/sbin/telnetd -l /bin/ash

Test if telnet server is working: telnet to the router

Note: this hack may not work on all firmware versions, I have tested it successfully on 2.21b. (Does not work on 2.22f, 2.24g and 2.26c: A link to busybox exists, but busybox was compiled without this plugin).